The present invention relates to a semiconductor light receiving element and a method of manufacturing the same, and in particular, to a semiconductor light receiving element having high-speed and highly efficient light-receiving characteristics and a method for manufacturing the same.
Conventionally, a semiconductor light receiving element formed from a semiconductor element converting light signals into electric signals has been known.
FIG. 7 is a perspective view showing a structure of an end face refracting type semiconductor light receiving element as such a general semiconductor light receiving element.
FIG. 8 is a cross-sectional view showing a structure of the end face refracting type semiconductor light receiving element as such a general semiconductor light receiving element.
Namely, as shown in FIG. 7 and FIG. 8, this end face refracting type semiconductor light receiving element is structured such that a running layer 2 formed from i-InP is formed on a substrate 1 formed from n+-InP.
On this running layer 2, a light absorbing layer 3 formed from p-InGaAs, a block layer 4 formed from p+-InGaAsP, and a contact layer 5 formed from p+-InGaAs are formed.
A p electrode 6 is mounted on the top surface of the contact layer 5.
Further, an n electrode 7 is mounted on the bottom surface of the above-described substrate 1.
Moreover, polyimide 8 is formed at one portion of the side surfaces of the running layer 2, the light absorbing layer 3, the block layer 4, and the contact layer 5, and at the bottom surface of the p electrode 6, in order to reduce the capacitance.
As shown in FIG. 8, in the end face refracting type semiconductor light receiving element, light is made incident on an inclined end face 1a of the substrate 1.
This incident light is refracted at the end face 1a, and thereafter, is made incident on the light absorbing layer 3 via the running layer 2.
Then, the incident light is absorbed at the light absorbing layer 3, and thereafter, is photoelectrically converted into electrons and positive holes.
Here, a predetermined reverse bias voltage is applied between the p electrode 6 and the n electrode 7.
FIG. 9 is a diagram showing a band-diagram of the end face refracting type semiconductor light receiving element at the time of applying the reverse bias voltage.
Next, by using this band diagram, the principles of operation of the end face refracting type semiconductor light receiving element will be considered in detail.
As described above, the incident light is absorbed at the light absorbing layer 3, and thereafter, the incident light is photoelectrically converted into electrons and positive holes. As a result, electrons 9 are generated at a conduction band, and positive holes 10 are generated at a valence band.
Generally, when the light absorbing layer 3 is not doped, because the mass of the positive holes 10 at the valence band is large, it is difficult for the positive holes 10 to move if a large bias voltage is not applied. Therefore, it is difficult for the positive holes 10 to be taken as electric current.
As a result, in the semiconductor light receiving element, a large bias voltage must be applied, and it is generally known that it is easy for heat destruction, which is due to the Joule heat provided by the product of the applied voltage and the flowing electric current being large, to arise.
In order to overcome such a problem, in the semiconductor light receiving element, a structure such as the following in which the light absorbing layer 3 is doped to p-type has been reported.
Namely, although an internal electric field does not exist in the light absorbing layer 3, because the positive holes 10 are the majority carrier, the movement thereof is fast regardless of the fact that the mass thereof is large. The positive holes 10 move to the contact layer 5 formed from p+-InGaAs, and thereafter, are output to the exterior via the p electrode 6.
On the other hand, the electrons 9, which are the minority carrier at the conduction band, move to the running layer 2 by diffusing in the light absorbing layer 3 in which no internal electric field exists.
Because an internal electric field exists in the running layer 2, the electrons 9 which have reached this region reach the substrate 1 formed from n+-InP at a high speed by drifting due to the internal electric field, and are taken out to the exterior via the n electrode 7.
Here, the block layer 4 formed from p+-InGaAsP blocks the electrons 9 such that the electrons 9 generated at the light absorbing layer 3 do not flow toward the contact layer 5 formed from p+-InGaAs.
In this way, because the carriers running at the running layer 2 which is a non-doped layer are only the electrons 9, the semiconductor light receiving element in which the light absorbing layer 3 is doped to p-type is called a unitraveling carrier photodiode (hereinafter, called UTC-PD) (refer to Jpn. Pat. Appln. KOKAI Publication No. 9-275224).
However, in the above-described UTC-PD, there are still problems to be solved as follows.
Namely, because the incident light is absorbed at the light absorbing layer 3, there is the need to make the thickness of the light absorbing layer 3 thick in order to efficiently convert the light incident on the unitraveling carrier type semiconductor photodiode from the exterior into the electrons 9 and the positive holes 10.
However, as described above, in the UTC-PD, an internal electric field does not exist in the light absorbing layer 3 doped to p-type, and the electrons 9 which are the minority carrier move only by diffusion in the light absorbing layer 3.
In this case, although the mass of the electrons 9 is light, the diffusion length of the electrons 9 generally is not long.
Therefore, in such a unitraveling carrier type semiconductor photodiode, if the light absorbing layer 3 is made thicker than the diffusion length of the electrons 9 in order to increase the efficiency of the light incident from the exterior being converted into electric current, the electrons 9 cannot reach the running layer 2, and the desired high-speed response cannot be obtained.
Moreover, in such a unitraveling carrier type semiconductor photodiode, light cannot be newly absorbed because the electrons 9 are not able to move and accumulate at the conduction band. As a result, the efficiency of the light incident from the exterior being converted into the electrons 9 and the positive holes 10 also deteriorates.
Namely, in the UTC-PD, due to the limit of the thickness of the light absorbing layer 3 doped to p-type, if the thickness is made thin in order to obtain a high-speed response, the efficiency of the light being converted into the electrons 9 and the positive holes 10 deteriorates. Conversely, if the thickness is made thick in order to obtain a high conversion efficiency, the high-speed response cannot be obtained. Therefore, there is the problem that it is difficult to realize a semiconductor light receiving element having both of two characteristics which are high speed and high efficiency.
Incidentally, in order to realize a response speed of 50 GHz or more with excellent reproducibility, the limit of the thickness of the light absorbing layer 3 is about 0.3 xcexcm from the standpoint of the diffusion length of the electrons 9.
Namely, this is because the problem arises that, if the thickness of the light absorbing layer 3 is made to be thick to, for example, about 0.6 xcexcm in order to increase the efficiency of converting the incident light into the electrons 9 and the positive holes 10, the operation frequency markedly deteriorates since the thickness becomes much greater than about 0.3 xcexcm which is the diffusion length of the electrons 9 described above.
Moreover, in the UTC-PD, also when the thickness of the light absorbing layer 3 is thinner than the diffusion length of the electrons, if the incident light amount of the light is low, the repulsion arising among the generated electrons is small. Therefore, the electrons cannot utilize the repulsion.
As a result, it has been understood that the problem of the frequency response characteristic deteriorating arises in UTC-PDs.
Namely, this is because, when the incident light amount of the light is low, there is hardly repulsion among the electrons because the number of generated electrons is small, and the electrons generated in the light absorbing layer 3 move purely by diffusion.
As a result, in the UTC-PD, when the incident light amount of the light is small, the frequency response characteristic deteriorates.
Namely, in the UTC-PD, because a high-speed response is possible only in cases of a large incident light amount, there is the limitation on use that an applicable system must be able to supply a large incident light amount to that UTC-PD.
In a general optical fiber communication system, due to absorption in an optical fiber, insertion loss of an optical device for use in a communication system or the like, the strength of the light entering light receiving element is weak.
Accordingly, in order to use the UTC-PD, there are problems from the standpoint of the cost of the communication system as well, such as there is the need to use a fiber amplifier before leading light to the photodiode, and the like.
The present invention has been achieved in consideration of circumstances such as those described above, and an object of the present invention is to provide a semiconductor light receiving element in which by interposing a spacer layer for acceleration between a plurality of light absorbing layers, high-speed movement of electrons at a light absorbing layer is possible, and a thick light absorbing layer on the whole can be used, and which is high-speed and highly efficient regardless of the magnitude of the incident light amount, and to provide a method of manufacturing the same.
In order to achieve the above object, according to an aspect of the present invention, there is provided a semiconductor light receiving element comprising:
an n electrode (7);
an n-type semiconductor doped layer or a non-doped layer (2) provided above the n electrode;
a semiconductor light absorbing layer provided above the n-type semiconductor doped layer or the non-doped layer;
a p-type semiconductor doped layer (4, 5) provided above the semiconductor light absorbing layer; and
a p electrode (6) provided above the p-type semiconductor doped layer,
wherein the semiconductor light absorbing layer has at least two layer portions (11, 13) doped to p-type, and a spacer layer (12) for acceleration which is formed from a semiconductor material sandwiched by the two layer portions and which makes electrons and positive holes generated by incident light being absorbed at the semiconductor light absorbing layer accelerate and run.
According to a second aspect of the present invention, there is provided a semiconductor light receiving element according to the first aspect, wherein the spacer layer is formed from a non-doped type semiconductor material.
According to a third aspect of the present invention, there is provided a semiconductor light receiving element according to the first aspect, wherein the spacer layer is formed from a semiconductor material doped to n-type.
According to a fourth aspect of the present invention, there is provided a semiconductor light receiving element according to the first aspect, wherein the spacer layer is formed from a semiconductor material weakly doped to p-type to an extent that an internal electric field exists.
According to a fifth aspect of the present invention, there is provided a semiconductor light receiving element according to the first aspect, wherein the spacer layer is formed from a multi-quantum well structure in which a plurality of semiconductor materials are combined.
According to a sixth aspect of the present invention, there is provided a semiconductor light receiving element according to the first aspect, wherein the semiconductor light absorbing layer is set such that band-gap wavelengths of the respective two layer portions doped to p-type become longer from the p electrode side toward the n electrode side.
According to a seventh aspect of the present invention, there is provided a semiconductor light receiving element according to the first aspect, wherein the semiconductor light absorbing layer is set such that a band-gap wavelength of at least one of the two layer portions doped to p-type becomes longer in an inclined manner from the p electrode side toward the n electrode side.
According to an eighth aspect of the present invention, there is provided a semiconductor light receiving element according to the first aspect, wherein the semiconductor light absorbing layer is set such that a dopant concentrations of at least one of the two layer portions doped to p-type becomes lower from the p electrode side toward the n electrode side.
According to a ninth aspect of the present invention, there is provided a semiconductor light receiving element according to the fourth aspect, wherein the semiconductor light absorbing layer is set such that a dopant concentration of at least one of the two layer portions doped to p-type and a dopant concentration of the spacer layer formed from a semiconductor material weakly doped to p-type to an extent that the internal electric field exists become lower from the p electrode side toward the n electrode side.
According to a tenth aspect of the present invention, there is provided a semiconductor light receiving element according to the first aspect, wherein the semiconductor light absorbing layer is weakly doped such that a dopant concentration of at least one of the two layer portions doped to p-type makes an internal electric field exist.
In order to achieve the above object, according to an eleventh aspect of the present invention, there is provided a method of manufacturing a semiconductor light receiving element comprising:
forming an n electrode (7);
forming an n-type semiconductor doped layer (2) or a non-doped layer above the n electrode;
forming a semiconductor light absorbing layer above the n-type semiconductor doped layer or the non-doped layer;
forming a p-type semiconductor doped layer (4, 5) above the semiconductor light absorbing layer; and
forming a p electrode (6) above the p-type semiconductor doped layer,
wherein the formation of the semiconductor light absorbing layer is performed by forming at least two layer portions (11, 13) doped to p-type, and
forming a spacer layer (12) for acceleration which is formed from a semiconductor material sandwiched by the two layer portions and which makes electrons and positive holes generated by incident light being absorbed at the semiconductor light absorbing layer accelerate and run.
According to a twelfth aspect of the present invention, there is provided a method of manufacturing a semiconductor light receiving element according to the eleventh aspect, wherein the spacer layer is formed a non-doped type semiconductor material.
According to a thirteenth aspect of the present invention, there is provided a method of manufacturing a semiconductor light receiving element according to the eleventh aspect, wherein the spacer layer is formed from a semiconductor material doped to n-type.
According to a fourteenth aspect of the present invention, there is provided a method of manufacturing a semiconductor light receiving element according to the eleventh aspect, wherein the spacer layer is formed from a semiconductor material weakly doped to p-type to an extent that an internal electric field exists.
According to a fifteenth aspect of the present invention, there is provided a method of manufacturing a semiconductor light receiving element according to the eleventh aspect, wherein the spacer layer is formed from a multi-quantum well structure in which a plurality of semiconductor materials are combined.
According to a sixteenth aspect of the present invention, there is provided a method of manufacturing a semiconductor light receiving element according to the eleventh aspect, wherein the semiconductor light absorbing layer is formed such that band-gap wavelengths of the respective two layer portions doped to p-type become longer from the p electrode side toward the n electrode side.
According to a seventeenth aspect of the present invention, there is provided a method of manufacturing a semiconductor light receiving element according to the eleventh aspect, wherein the semiconductor light absorbing layer is formed such that a band-gap wavelength of at least one of the two layer portions doped to p-type becomes longer in an inclined manner from the p electrode side toward the n electrode side.
According to an eighteenth aspect of the present invention, there is provided a method of manufacturing a semiconductor light receiving element according to the eleventh aspect, wherein the semiconductor light absorbing layer is formed such that a dopant concentration of at least one of the two layer portions doped to p-type becomes lower from the p electrode side toward the n electrode side.
According to a nineteenth aspect of the present invention, there is provided a method of manufacturing a semiconductor light receiving element according to the fourteenth aspect, wherein the semiconductor light absorbing layer is formed such that a dopant concentration of at least one of the two layer portions doped to p-type and a dopant concentration of the spacer layer formed from a semiconductor material weakly doped to p-type to the extent that the internal electric field exists, become lower from the p electrode side toward the n electrode side.
According to a twentieth aspect of the present invention, there is provided a method of manufacturing a semiconductor light receiving element according to the eleventh aspect, wherein the semiconductor light absorbing layer is weakly doped such that a dopant concentration of at least one of the two layer portions doped to p-type makes an internal electric field exist.